Different types of chemistry can be employed to formulate impression materials. Mostly used are polyether impression materials cured by a cationic ring-opening polymerization of aziridines (e.g. Impregum™, 3M ESPE), polysiloxanes cured via a hydrosylation reaction (e.g. Aquasil™, Dentsply) polysiloxanes cured via a condensation mechanism (e.g. Xantropren™, Heraeus Kulzer), mixtures of polyethers and siloxanes cured via a hydrosylation mechanism (e.g. Senn™, GC) or polyethers cured via a condensation mechanism (e.g. P2™, Heraeus Kulzer).
Dental impression materials are typically reactive systems that cure in the patient's mouth in order to record the oral situation. Mostly dental impression materials are delivered as two pastes, a base paste and a catalyst paste, which are mixed together prior to their application. With the help of a dental tray and a syringe-type device for syringing material around the prepared tooth or teeth the mixed paste is delivered in the patient's mouth. Usually the cured material can be removed between one and six minutes after tray seating. The finished impression is used either to make a provisional restoration or it is to be casted to form a casting model and make the final restoration in the dental laboratory.
The base paste and the catalyst paste of the dental impression materials as well as their mixture in the state before the curing should have shear-thinning properties. Shear-thinning is the feature of a fluid to show a decreasing viscosity at increasing shearing force. Thus, shear thinning ensures a low viscosity at a high shear rate and a high viscosity at a low shear rate. This behaviour can sometimes be desirable. In a first aspect it may simplify the mixing process of the base paste with the catalyst paste. In a second aspect the risk of dropping under gravity of the mixed impression material is reduced (e.g. dropping of the material from the upper jaw). Further, it should nicely flow to gingiva, into small gaps and around teeth when syringed or when the tray is seated. Also under the compression of a delivery device like a hand dispenser (e.g. supplied by SulzerMixpac) or an electronic mixing device (e.g. Pentamix™, 3M ESPE) the material (base paste, catalyst paste and mixture of the pastes) should flow nicely. Using the terminology of rheometry the paste can be characterized by its viscosity at different shear rates. At low shear rates a high viscosity is desired whereas at high shear rates a low viscosity is desirable.
In the prior art, to achieve these shear-thinning properties for dental impression materials two major classes of rheology modifiers are used, highly dispersed silica and fats.
For instance, DE 43 21 257 A1 suggests the use of pyrogenic and/or precipitated silica with a BET surface of 25 to 600 m2/g to improve polymerizable polyether materials.
In DE 197 11 514 A1 (also published as U.S. Pat. No. 6,127,449) teaches to use 5 to 20 wt.-% of a triacylglyceride of a non-animal source as rheology modifier for impression material based on polymerizable polyether material.
DE 195 17 962 A1 (also published as U.S. Pat. No. 6,040,354) discloses a material based on polysiloxanes containing at least one wax except paraffin or microwax. The method to produce this material includes heating the mixture above the melting point of the wax, to emulsify the mixture and to cool the emulsion rapidly. Only after storing at room temperature for about 12 hours the fillers and rheological modifiers can be incorporated into the material with a separate kneading step.
EP 1 563 823 A2 (also published as US 2005/250871) discloses dental material based on alkoxysilylfunctionalized polyethers with a catalyst. As thixotropic agents different substances can be added.
U.S. Pat. No. 5,569,691 relates to a rubber-elastic composition comprising a vulcanizable polyether material and at least one hydrophilic nature imparting agent. In Example 4 (Comparative Example) a composition is described containing besides an aziridino polyether a certain amount of a substance designated Telamide™. The properties of this substance have been tested in the experimental section of the present invention.
US 2006/47063 describes compositions containing at least one silane-terminated polyether derivative. As fillers are used organic fillers like hydrogenated castor oil or castor oil derivatives, polyamides, polyesters, paraffins, waxes and fats. The compositions mandatory comprise water.
A disadvantage of using waxes of the state of the art as rheological modifiers is sometimes the complicated production process which involves a melting step in a vessel followed by a shock-cooling step using a shock cooling roller in which the liquid components of the paste and the molten triglyceride are subjected. Finally, after storing the material for some hours at about room temperature, a further kneading step in a kneader is necessary to incorporate the fillers and rheological modifiers.
For many materials of the state of the art, highly dispersed silica or more general highly dispersed oxides are used. These compounds optionally may be surface treated.
Highly dispersed silica is relatively cheep. Also a wide variety of materials differing in surface area and surface treatment is available. Examples are Aerosil™ (Degussa) or HDKH™ (Wacker). In general the highly dispersed silica might be reacted with hydrophilic, hydrophobic, reactive or unreactive substances in order to achieve the desired surface treatment.
A disadvantage of using highly dispersed silica types in dental formulations together with the reactive monomers or pre-polymers that form dental impression materials is the relatively low shelf life of the components of the impression material prior mixing. This in particular refers to the base paste. Si—OH groups which are located at the surface are highly reactive. It is known to the specialist that it is impossible to react all Si—OH groups at the surface of highly dispersed silica. Remaining Si—OH groups are responsible for the compromised shelf life of the reactive systems.
For example, Si—OH groups are incompatible with aziridines, the reactive group of e.g. aziridino-polyethers that build one group of reactive monomers or pre-polymers, since traces of acidic components cause early polymerization.
In conjunction with residual water which is always present in fillers and in polyethers e.g. in monomers, pre-polymers or surfactants, Si—OH groups are also incompatible with Si—H functions that form the reactive group used in the impression materials cured via hydroxylation. The loss of Si—H functions is observed which results in reduced speed of cure, reduced cross link density accompanied by lower mechanical values and hydrogen release which might be an issue for materials delivered in foil bags.
In conjunction with residual water Si—OH groups are also incompatible with Si(OR)x[x=1, 2, 3; R=alkyl preferably Me or Et] functions that form the reactive group in the materials cured via condensation reaction. The systems tend to the loss of reactive functions and to pre-polymerization.